The manner in which teammates behave toward each other when playing sport could have important achievement-related consequences. However, this issue has received very little research attention. In this study, we investigated whether: (a) prosocial and antisocial teammate behavior predict task cohesion and burnout; and (b) positive and negative affect mediate these relationships. Two hundred and seventy-two (96 males; Mage = 21.86, SD = 4.36) team-sport players completed a multi-section questionnaire assessing the aforementioned variables. Structural equation modeling using maximum likelihood estimation indicated that prosocial teammate behavior positively predicted task cohesion and negatively predicted burnout, and these effects were mediated by positive affect. The reverse pattern of relationships was observed for antisocial teammate behavior: This behavior negatively predicted task cohesion and positively predicted burnout, and these effects were mediated by negative affect. Our findings underscore the importance of promoting prosocial and reducing antisocial behavior in sport and highlight the role of affect in explaining the identified relationships.